U.S. patent number 8,053,089 [Application Number 12/570,011] was granted by the patent office on 2011-11-08 for single layer bond coat and method of application.
This patent grant is currently assigned to General Electric Company. Invention is credited to Joseph G. Albanese, Stephen D. Dillon, Joshua Lee Margolies, Tamara Jean Muth.
United States Patent |
8,053,089 |
Margolies , et al. |
November 8, 2011 |
Single layer bond coat and method of application
Abstract
A protective coating system for metal components includes a
superalloy metal substrate, such as a component of a gas turbine. A
single layer bond coat is applied to the superalloy metal substrate
in a thermal spray process from a homogeneous powder composition
having a particle size distribution wherein about 90% of the
particles by volume are within a range of about 10 .mu.m to about
100 .mu.m. The percentage of particles within any 10 .mu.m band
within the range does not exceed about 20% by volume, and the
percentage of particles within any two adjacent 10 .mu.m bands
within the range does not deviate by more than about 8% by
volume.
Inventors: |
Margolies; Joshua Lee
(Niskayuna, NY), Albanese; Joseph G. (Rotterdam Junction,
NY), Muth; Tamara Jean (Ballston Lake, NY), Dillon;
Stephen D. (Duanesburg, NY) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
43259757 |
Appl.
No.: |
12/570,011 |
Filed: |
September 30, 2009 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20110076413 A1 |
Mar 31, 2011 |
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Current U.S.
Class: |
428/678; 428/687;
428/679; 428/632; 428/937; 428/621; 416/241R; 428/612 |
Current CPC
Class: |
C23C
4/134 (20160101); F01D 5/288 (20130101); C23C
4/073 (20160101); C23C 28/3455 (20130101); C23C
28/3215 (20130101); C23C 4/129 (20160101); Y10T
428/12993 (20150115); Y10T 428/12931 (20150115); Y10T
428/12535 (20150115); Y10T 428/12611 (20150115); Y10T
428/12937 (20150115); Y10S 428/937 (20130101); Y10T
428/12472 (20150115); F02F 3/12 (20130101) |
Current International
Class: |
B32B
15/04 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1939317 |
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Jul 2008 |
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EP |
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9943861 |
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Sep 1999 |
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WO |
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Other References
Bharat K, Pant, Vivek Arya, and B.S. Mann "Development of Low-Oxide
MCrAlY Coatings for Gas Turbine Applications" ; pp. 275-280.
Journal of Thermal Spray Technology, vol. 16(2), Jun. 2007. cited
by other .
International Search Report issued in connection with corresponding
EP Application No. 10179700 on Dec. 20, 2010. cited by
other.
|
Primary Examiner: Austin; Aaron
Attorney, Agent or Firm: Dority & Manning, P.A.
Claims
What is claimed is:
1. A protective coating system for metal components, comprising: a
superalloy metal substrate; a single layer bond coat applied to the
superalloy metal substrate, said bond coat applied in a thermal
spray process from a single homogeneous metallic powder composition
having a particle size distribution wherein: about 90% of the
particles by volume are within a range of about 10 .mu.m to about
100 .mu.m; the percentage of particles within any 10 .mu.m band
within the range does not exceed about 20% by volume; and the
percentage of particles within any two adjacent 10 .mu.m bands
within the range does not deviate by more than about 8% by
volume.
2. The system in claim 1, wherein said bond coat comprises the
following additional characteristics: a surface roughness of at
least about 300.mu. inch Ra; a density of at least about 90% of
theoretical density; and a bond coat to substrate tensile strength
of at least about 6.0 ksi.
3. The system as in claim 1, wherein said bond coat comprises the
following additional characteristics: a surface roughness of at
least about 400.mu. inch Ra; a density of at least about 95% of
theoretical density; and a bond coat to substrate tensile strength
of at least about 12.0 ksi.
4. The system as in claim 1, wherein said single layer bond coat is
applied in a thermal spray process having a particle velocity of at
least about 300 m/s.
5. The system as in claim 1, wherein said single layer bond coat is
applied in one of a high velocity oxy-fuel (HVOF) thermal spray
process or a high velocity air plasma spray (HV-APS) thermal spray
process.
6. The system as in claim 1, further comprising a ceramic thermal
barrier coat (TBC) applied over said single layer bond coat, and a
TBC to bond coat tensile strength that exceeds the cohesive
strength of the ceramic thermal barrier coat material.
7. The system as in claim 6, wherein the TBC to bond coat tensile
strength is at least about 4.0 ksi.
8. The system as in claim 1, wherein said bond coat powder
composition comprises MCrAlY alloy particles, where M is at least
one of iron, cobalt, or nickel.
9. The system as in claim 1, wherein said metal substrate is a
component of a gas turbine.
Description
FIELD OF THE INVENTION
The present invention relates generally to protective coatings
applied to metal substrates. More specifically, the invention is
directed to a single layer bond coat having the benefits of
conventional bi-layer bond coats, and to the related method for
application of such single layer bond coats.
BACKGROUND OF THE INVENTION
Higher operating temperatures for gas turbine engines are
continuously sought in order to increase their efficiency. However,
as operating temperatures increase, the temperature durability of
the engine components must correspondingly increase. Significant
advances in high temperature capabilities have been achieved
through the formulation of nickel and cobalt-based superalloys, and
through the development of oxidation-resistant overlay coatings
deposited directly on the surface of the superalloy substrate to
form a protective oxide scale during high temperature exposure.
Nonetheless, superalloys protected by overlay coatings often do not
retain adequate mechanical properties for components located in
certain sections of a gas turbine engine, such as the combustor and
augmentor. A common solution is to thermally insulate such
components in order to minimize their service temperatures. For
this purpose, thermal barrier coating (TBC) systems formed on the
exposed surfaces of high temperature components have found wide
use.
To be effective, TBC systems must have low thermal conductivity,
strongly adhere to the article, and remain adherent throughout many
heating and cooling cycles. The latter requirement is particularly
demanding due to the different coefficients of thermal expansion
between materials having low thermal conductivity and superalloy
materials typically used to form turbine engine components. TBC
systems capable of satisfying the above requirements generally
require a metallic bond coat deposited on the component surface,
followed by an adherent thermal barrier ceramic layer that serves
to thermally insulate the component. Various ceramic materials have
been employed as the thermal barrier layer, particularly zirconia
(ZrO.sub.2) stabilized by yttria (Y.sub.2O.sub.3), magnesia (MgO),
ceria (CeO.sub.2), scandia (Sc.sub.2O.sub.3), or another oxide.
The bond coat is typically formed from an oxidation-resistant
aluminum-containing alloy to promote adhesion of the ceramic layer
to the component and inhibit oxidation of the underlying
superalloy. Examples of prior art bond coats include overlay
coatings such as MCrAlY (where M is iron, cobalt and/or nickel),
and diffusion coatings such as diffusion aluminide or platinum
aluminide, which are oxidation-resistant aluminum-base
intermetallics. The bond coat is typically disposed on the
substrate by a thermal spray processes, such as vacuum plasma spray
(VPS) (also know as low pressure plasma spraying (LPPS)), air
plasma spray (APS), and high velocity oxy-fuel (HVOF) spray
processes.
Conventional bond coats are typically applied as a bi-layer
construction wherein a fine powder is first deposited on the
substrate to form a dense, low oxide layer. Commercially available
HVOF systems are typically used to deposit this layer. It is
generally recognized that conventional HVOF processes are sensitive
to particle size distributions, generally requiring finer particles
ranging from -45+10 .mu.m. The fine particle layer serves to
protect the substrate from oxidation and corrosion, but the low
surface roughness of the layer results in inadequate adhesion of
the ceramic material layer.
A coarse powder layer is then deposited over the fine powder layer
to achieve a desired degree of surface roughness for adequate
adhesion of the ceramic material. APS bond coating techniques are
often favored for the coarse powder layer due to lower equipment
cost and ease of application and masking. Adhesion of the ceramic
material layer to an APS bond coat is promoted by forming the bond
coat to have a surface roughness of about 200 microinches (about 5
.mu.m) to about 500 microinches (about 13 .mu.m) Ra (Arithmetic
Average Roughness (Ra) as determined from ANSI/ASME Standard
B461-1985).
Although APS-applied bond coats provide better TBC adhesion due to
their roughness, the coarse powder layer is generally unsuitable as
a protective coating system. The coarse powder layer is relatively
porous and prone to oxidation damage.
Thus, conventional bond coats are applied as a bi-layer in separate
processes with separate equipment configurations to achieve the
desired characteristics of a dense, low-oxide protective layer, and
the surface roughness of a coarse powder layer. This practice,
however, requires maintaining both powders in inventory, as well as
the different coating systems. The process is time consuming in
that it involves set up for two different processes, and can result
in rework of coated pieces due to equipment or powder mix-ups.
Accordingly, the art would benefit from an improved commercially
viable process for applying a single layer bond coat from a single
powder composition, with the bond coat having the desired
properties of conventional bi-layer bond coats.
BRIEF DESCRIPTION OF THE INVENTION
Aspects and advantages of the invention will be set forth in part
in the following description, or may be obvious from the
description, or may be learned through practice of the
invention.
The present invention provides a protective coating system for a
metal substrate, and is particularly suited for metal components of
a gas turbine engine. The system includes a superalloy metal
substrate having a single layer bond coat applied to the substrate.
The bond coat is applied in a thermal spray process, for example a
high velocity oxy-fuel (HVOF) process, from a homogeneous powder
composition that results in a bond coat having properties
comparable to bi-layer bond coats. The powder composition has a
particle size distribution wherein about 90% of the particles by
volume are within a range of about 10 .mu.m to about 100 .mu.m. The
particles are distributed relatively uniformly within the range in
that the percentage of particles within any 10 .mu.m band within
the range does not exceed about 20% by volume, and the percentage
of particles within any two adjacent 10 .mu.m bands within the
range does not deviate by more than about 8% by volume. The coating
system may also encompass a ceramic thermal barrier layer applied
to the single layer bond coat, or the bond coat may be the only
layer of the protective coating system.
The present invention also encompasses a method for forming a
protective coating system on a metal substrate. The method includes
applying a single layer bond coat to a superalloy metal substrate,
such as a Ni or Co based superalloy, in a thermal spray process,
for example an HVOF process, from a homogeneous powder composition
having a particle size distribution such that the resulting bond
coat has properties at least comparable to bi-layer bond coats.
About 90% by volume of the particles are within a range of about 10
.mu.m to about 100 .mu.m. The percentage of particles within any 10
.mu.m band within the range does not exceed about 20% by volume,
and the percentage of particles within any two adjacent 10 .mu.m
bands within the range does not deviate by more than about 8% by
volume. A single layer bond coat formed in accordance with the
present method may have a surface roughness of at least about 300
.mu.inch Ra, a density of at least about 90% of theoretical
density; and a bond coat to substrate tensile strength of at least
about 6.0 ksi. The bond coat powder composition may include MCrAlY
alloy particles, where M is at least one of iron, cobalt, or
nickel. In a further refinement of the method, a ceramic thermal
barrier layer is applied over the single layer bond coat, with a
thermal barrier layer to bond coat tensile strength that exceeds
the cohesive strength of the ceramic layer, regardless of the
morphology of the ceramic layer. This ceramic barrier layer may be
formed from, for example, commercially available yttria stabilized
ceramic coating particles.
These and other embodiments and features of the invention will be
described in greater detail in the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
A full and enabling disclosure of the present invention, including
the best mode thereof, directed to one of ordinary skill in the
art, is set forth in the specification, which makes reference to
the appended figures, in which:
FIG. 1 is a cross-sectional view of a conventional thermal barrier
coat protective system having a bi-layer bond coat;
FIG. 2 is a cross-sectional view of a single layer bond coat
applied to a metal substrate in accordance with aspects of the
invention;
FIG. 3 is a cross-sectional view of a thermal barrier coat system
having a single layer bond coat in accordance with aspects of the
invention;
FIG. 4 is a perspective view of a conventional gas turbine blade
configuration;
FIG. 5 is a plot of the particle size distribution profile for
various powder compositions;
FIGS. 6 through 8 are micrograph pictures of test samples having a
first embodiment of a single layer bond coat in accordance with
aspects of the invention; and
FIGS. 9 through 11 are micrograph pictures of test samples having a
second embodiment of a single layer bond coat in accordance with
aspects of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Reference now will be made in detail to embodiments of the
invention, one or more examples of which are illustrated in the
drawings. Each example is provided by way of explanation of the
invention, not limitation of the invention. In fact, it will be
apparent to those skilled in the art that various modifications and
variations can be made in the present invention without departing
from the scope or spirit of the invention. For instance, features
illustrated or described as part of one embodiment, can be used
with another embodiment to yield a still further embodiment. Thus,
it is intended that the present invention covers such modifications
and variations as come within the scope of the appended claims and
their equivalents.
As previously discussed, thermal barrier coating (TBC) systems are
often used to improve the efficiency and performance of metal parts
which are exposed to high temperatures, such as nozzles, buckets,
shrouds, airfoils, and other gas turbine components. The combustion
gas temperatures present in conventional gas turbines are
maintained as high as possible for operating efficiency, and
turbine combustion components and other elements of the engine are
usually made of alloys which can resist the high temperature
environment, e.g., superalloys, which have an operating temperature
limit of about 1000-1150 degrees Celsius. The TBC systems
effectively increase the operating temperature of the turbine by
maintaining or reducing the surface temperature of the alloys used
to form the various engine components.
The TBC systems are also critical for protecting the surfaces of
various turbine components. Referring to FIG. 1, most conventional
TBC systems are dual-layer systems that include a ceramic-based top
layer 38 deposited over a denser, oxidation-resistant bi-layer bond
coat 32. The ceramic material is typically a material like zirconia
(zirconium oxide), which is usually chemically stabilized with
another material such as yttria. The bond coat 32 is applied to a
metal substrate 40 as a bi-layer construction wherein a fine powder
is first deposited on the substrate to form a dense, low oxide
layer 34. A coarse powder layer 36 is then deposited over the fine
powder layer to achieve a desired degree of surface roughness for
adequate adhesion of the ceramic material 38.
Referring to FIG. 2, the present invention relates to a protective
coating system 50 having an improved, single layer bond coat (SLBC)
54 applied to a metal substrate 40. Although the SLBC 54 will
typically form the initial layer in a TBC system, it should be
appreciated that a bond coat 54 in accordance with the present
invention may also be used as a stand-alone protective overlay
coating on any manner of metallic substrate, i.e., without a
ceramic top layer, as depicted in FIG. 2. FIG. 3 depicts a
protective coating system 50 in accordance with the invention that
includes a ceramic layer 38 applied over the SLBC 54.
Single layer bond coatings 54 in accordance with the invention may
be applied to components of a gas turbine, as discussed above, or
used in other environments, such as selected components of diesel
or other types of internal combustion engines. FIG. 4 is provided
for purposes of illustrating an environment in which the present
invention is particularly useful, and depicts a conventional gas
turbine blade configuration 10. A plurality of the blades 10 are
attached to an annular rotor disk (not shown) in a gas turbine.
Blade 10 includes an airfoil 12, having pressure and suction sides
14, 16, and leading and trailing edges 18 and 20. The lower part of
the airfoil terminates with base 22. Base 22 includes a platform
24, in which the airfoil can be rigidly mounted in an upright
position, i.e., substantially vertical to the top surface 25 of the
platform. The base further includes a dovetail root 26, attached to
the underside of the platform, for attaching blade 10 to the rotor.
The airfoil 12 is at least one component that typically requires a
thermal barrier coating.
The SLBC 54 is applied to any manner of metal substrate 40 in a
thermal spray process from a homogeneous powder composition having
a particle size distribution that provides the SLBC 54 with
comparable characteristics of a bi-layer bond coat. In particular,
the SLBC 54 has the density and low oxide content of a fine powder
layer comparable to layer 34 of FIG. 1, and the surface roughness
of a coarse powder layer comparable to layer 36 of FIG. 1.
Referring to the particle size distribution graph of FIG. 5, a
homogeneous powder composition used in the thermal spray process to
apply the SLBC 54 has the particle size characteristics of for
example, graphs C, D, or E in that about 90% of the particles by
volume are within a range of about 10 .mu.m to about 100 .mu.m. In
addition, the percentage of particles within any 10 .mu.m band
within the range does not exceed about 20% by volume, and the
percentage of particles within any two adjacent 10 .mu.m bands
within the range does not deviate by more than about 8% by volume.
For example referring to an ideal distribution graph C, it is seen
that the particles within any 10 .mu.m (i.e., 20 to 30 .mu.m band,
or 30 to 40 .mu.m band, or 35 to 45 .mu.m band) do not exceed about
13% by volume of the composition, and such percentage does not
deviate across the range. In other words, the percentage of
particles within the band of 20 to 30 .mu.m is the same as the
percentage of particles within the band of 30 to 40 .mu.m, and so
forth.
The graph C in FIG. 5 is considered "ideal" because of its flat,
truncated profile wherein the percentage of particles within the 10
.mu.m bands is the same across the stated range (i.e., range of
about 10 .mu.m to about 100 .mu.m). However, this profile may not
be economically feasible or otherwise attainable with blends or
mixtures of commercially available powders. A more realistic
particle size distribution may be reflected by, for example, graph
D. This profile has a "bi-modal" aspect in that distinct fine and
coarse particle peaks are identifiable, yet the overall profile
still satisfies the requirements set forth above.
Graph A in FIG. 5 illustrates a typical particle size distribution
curve for conventional fine particles used to form an initial layer
34 (FIG. 1) in conventional TBC systems, and is provided for
purposes of comparison with curves for powder compositions in
accordance with the present invention Conventional fine powders
have a particle size distribution range of generally about -53+22
.mu.m (d10 percentile of approximately 22 .mu.m; d90 percentile of
approximately 53 .mu.m). Commercial HVOF powders are typically in
the range of about -45+10 .mu.m. Graph B is a typical particle size
distribution curve for coarse powders used to form the second layer
36 of conventional bond coats 32 (FIG. 1), and is also provided for
comparative purposes. These coarse powders have a particle size
distribution range of about -100+44 .mu.m (d10 percentile of
approximately 44 .mu.m; d90 percentile of approximately 100
.mu.m).
Graph E in FIG. 5 is provided as an example of another type of
powder composition that falls within the scope of the present
invention. This graph has a profile that reflects a generally
continuously changing profile similar to a bell-curve that still
satisfies the requirements set forth above. It should be
appreciated that any manner of particle size distribution curve is
possible that satisfies the requirements of the present invention,
and that the invention is not limited to any particular curve or
distribution profile that satisfies the stated requirements.
The SLBC 54 formed from a powder composition as described above has
a surface roughness of at least about 300 .mu.inch Ra (Arithmetic
Average Roughness (Ra) as determined from ANSI/ASME Standard
B461-1985). In particular embodiments, the surface roughness will
be at least about 400 .mu.inch Ra. The rough surface serves to
ensure good adhesion between the bond coat and a subsequently
applied thermal barrier material. It should be appreciated that the
surface roughness value of the SLBC is not an issue when the SLBC
is used as the only layer in the protective coating system, i.e., a
ceramic thermal barrier material layer is not applied over the
SLBC.
Single layer bond coats 54 according to the present process may be
formed having any suitable thickness. Typical bond coats in a
bi-layer coating system are typically within a range of about 250
.mu.m to about 500 .mu.m. A SLBC 54 in accordance with the present
invention may not need to be as thick as these conventional bond
coats, and may have a thickness less than conventional bond coats,
for example, of about 125 .mu.m, or 200 .mu.m. It is believed that
a 200 .mu.m SLBC will produce the equivalent life of a 350 .mu.m
bi-layer bond coat.
The SLBC 54 will also have a density of at least about 90% of
theoretical density, and in particular embodiments, at least about
95% of theoretical density.
The SLBC 54 also has a bond coat to substrate tensile strength of
at least about 6.0 ksi, and in particular embodiments, at least
about 12.0 ksi.
The SLBC 54 is applied in a thermal spray process having a particle
velocity of at least about 400 m/s. Various techniques are
available for measuring particle velocity downstream from the
plasma gun exit, using a variety of sensor systems. As a
non-limiting example, measuring systems for determining particle
velocity and particle velocity distribution are described in U.S.
Pat. No. 6,862,536 (Rosin). One example of an on-line particle
monitoring and measurement device which is commercially available
is the DPV-2000 system, available from Tecnar Automation Ltd,
Montreal, Canada (http://www.tecnar.com/).
Although it is generally held that conventional high velocity
oxy-fuel (HVOF) thermal spray systems are sensitive to particle
size distributions (generally requiring finer particles ranging
from -45+10 .mu.m), the present inventors have found that such HVOF
systems may be used for the protective coating system and
methodology of the present invention. By carefully monitoring and
adjusting the HVOF thermal spray parameters, a single layer bond
coat is achievable from a powder composition as described herein
that is dense and relatively oxide-free, yet has the surface
roughness and porosity required for good adhesion of a ceramic
material layer. For example, the combustion ratio in a HVOF process
for purposes of the present invention should be less than about
0.29, and desirably within a range of about 0.27 to about 0.29.
This combustion ratio with the powder composition discussed above
yields satisfactory deposition rates.
With respect to deposition rates, the relationship of pounds of
powder per mil of coating per square unit of area coated is an
objective standard. A deposit efficiency is desirable that produces
a satisfactory coating without excess wastage of powder. A baseline
parameter may first be established, for example 0.13 lbs. per mil
of coating per square foot of surface coating. The combustion ratio
may then be adjusted from a low baseline value of, for example,
0.235, until the plume temperature reaches a limit indicative of
excessive oxide in past experience with similar powder chemistries.
With the increased combustion ratio, an increased deposit rate
efficiency results of about 0.08 lbs. of powder per square foot of
area coated to a thickness of about 1 mil. A further increase of
the combustion ratio so that even less powder is required may lead
to unacceptable levels of oxide in the coating. A deposition rate
range of about 0.15 to about 0.08 lbs/mil/ft.sup.2 at a combustion
ratio that does not produce unacceptable oxides in the coating may
be desired for purposes of the SLBC 54.
Examples of the other steps and process parameters that may be
adjusted to achieve a SLBC 54 in accordance with the present
invention include: cleaning of the surface prior to deposition;
grit blasting to remove oxides; substrate temperature; other plasma
spray parameters such as spray distance (gun-to-substrate);
selection of the number of spray-passes, powder feed rate, torch
power, plasma gas selection; angle-of-deposition, post-treatment of
the applied coating; and the like.
Another suitable thermal spray process is a high velocity air
plasma spray (HV-APS) process wherein particle velocity is
maintained in the range of about 300 m/s to about 700 m/s. In some
specific embodiments, the velocity is at least about 450 m/s, and
may be about 600 m/s. These particle velocities are substantially
greater than the typical velocities used in conventional APS
systems (in the range of about 150-250 m/s). For a HV-APS system, a
conventional APS system can be modified to effectively increase the
plasma velocity and hence, the particle velocity. In general,
modification of the APS system in this instance involves the
selection of different configurations of anode nozzles which fit
into the plasma spray guns, and commercial APS guns equipped with
high-velocity anode nozzles can be employed to carry out the high
velocity air plasma spray (HV-APS) process. Non-limiting examples
include the 7 MB (or 9 MB, or 3 MB) plasma spray gun equipped with
the 704 high velocity nozzle, available from Sulzer Metco, Inc.
Another example is the SG100 plasma spray gun, operated in the
"Mach 2" mode, available from Praxair Surface Technologies, Inc.
These conventional APS gun systems may be operated in a power range
of, for example, 30-50 KW.
The powder composition for the SLBC 54 may comprise MCrAlY alloy
particles, where M is at least one of iron, cobalt, or nickel.
The resulting SLBC 54 has a density of at least 90% of theoretical
density, and more particularly about 95% density. These densities
reflect a decreased oxide content in the bond coating that greatly
increases the effective TBC life and the substrate life against
oxidation. Decreased oxide content in the bond coat (as reflected
by an increased density) inhibits detrimental growth of thermally
grown oxide (TGO) at the interface of the bond coat and ceramic
coat during service of the component. It is generally accepted that
TGO accelerates TBC failures, such as cracking, delamination, and
spalling.
Referring to FIG. 3, the protective coating system 50 of the
present invention may also encompass application of a thermal
barrier material 38 applied over the bond coat 54, which may
include any of various known ceramic materials, such as zirconia
(ZrO.sub.2) stabilized by yttria (Y.sub.2O.sub.3), magnesia (MgO),
ceria (CeO.sub.2), scandia (Sc.sub.2O.sub.3), or another oxide.
Commercially available yttria stabilized ceramic coating particles
may be used for the TBC material, for example, Sulzer Metco 240NS 8
wt % yttria stabilized zirconia powder having a particle size
distribution range of about -11+125 .mu.m (d10 percentile of
approximately 11 .mu.m; d90 percentile of approximately 125 .mu.m),
or Sulzer Metco 240NA powder having a particle size distribution
range of about -97+25 .mu.m. The ceramic barrier material 38 may be
deposited by any suitable known technique, such as by physical
vapor deposition (PVD) techniques, particularly electron beam
physical vapor deposition (EBPVD), or conventional APS techniques.
Desirably, the coating system 50 produces a thermal barrier coat 38
to bond coat 54 tensile strength that exceeds the cohesive strength
of the ceramic layer, regardless of the morphology of the ceramic
layer. For example, for a dense vertically cracked ceramic layer, a
tensile strength of at least about 4.0 ksi., and at least about 5.0
ksi. in certain embodiments, may be desired. The thickness of the
ceramic barrier coating 38 will depend on the end use of the part
being coated. The thickness is usually in the range of about 100
microns to about 2500 microns. In some specific embodiments for end
uses such as airfoil components, the thickness is often in the
range of about 125 microns to about 750 microns.
Gas turbine component parts are exemplified as the "metal
substrate" in this patent specification. It should be appreciated,
however, that other types of components could serve as metal
substrates for bond coats in accordance with the invention. As one
example, the substrate may be the piston head of a diesel engine,
or other automotive parts. It should be readily appreciated that
the invention is not limited to any particular type of metal
substrate or component.
EXAMPLES
The following examples are merely illustrative, and should not be
construed to be any sort of limitation on the scope of the claimed
invention.
Example 1
A first (Sample A) bi-modal MCrAlY powder composition having a
particle size distribution generally in accordance with Graph D of
FIG. 5 was evaluated for microstructure properties, surface
roughness, and deposit efficiency as compared to a conventional
bi-layer bond coat. An initial bond coat test button sample was
thermally sprayed in a HVOF process with a Sulzer Metco DJ 2600
system. This baseline sample is illustrated in the micrograph
picture of FIG. 6. The spray process parameters were adjusted to
optimize deposit efficiency, as discussed above. In particular,
baseline spray parameters included a combustion ratio of about
0.235 and a low deposit rate, which produced an inefficient process
wherein essentially more of the powder was landing on the floor of
the chamber than was adhering to the component. Using process
monitoring diagnostics, the combustion ratio was increased until
the plume temperature reached a limit indicative of excessive oxide
in past experience with similar powder chemistries. This new
parameter produced a combustion ratio with a significant
improvement in efficiency of powder sticking to the component. The
deposition rate was adjusted to between about 0.08 to about 0.1
lbs/mil/ft.sup.2 at a combustion ratio (Oxygen/Fuel ratio) of about
0.28 (resulting in a deposition rate of about 0.68 mils/pass), to
produce the adjusted test button sample shown in the micrograph
picture of FIG. 7. This adjusted test sample was inspected for
microstructure properties and satisfied the density requirement of
at least about 90% of theoretical, and had a measured surface
roughness of about 490 Ra. The sample exhibited a bond coat to
substrate tensile strength exceeding 12.0 ksi. A ceramic thermal
barrier coat was added to the bond coat of FIG. 7 in an APS process
from a yttria stabilized ceramic powder to produce the test sample
shown in the micrograph of FIG. 8. This test sample exhibited a
ceramic thermal barrier coat to bond coat tensile strength of about
5.1 ksi.
Example 2
A second (Sample B) bi-modal powder composition having a particle
size distribution generally in accordance with Graph D of FIG. 5
was used to produce test buttons as described above with respect to
Sample A. The baseline sample is illustrated in the micrograph
picture of FIG. 9. The deposition rate was adjusted to about 0.53
mils/pass at a combustion ratio (Oxygen/Fuel ratio) of about 0.28
to produce the adjusted test button sample shown in the micrograph
picture of FIG. 10. This adjusted test sample was inspected for
microstructure properties and satisfied the density requirement of
at least about 90% of theoretical, and had a surface roughness of
about 452 Ra. The sample exhibited a bond coat to substrate tensile
strength exceeding 12.0 ksi. The same ceramic thermal barrier
material was added to the adjusted test sample to produce the test
sample shown in the micrograph of FIG. 11. This sample exhibited a
ceramic thermal barrier coat to bond coat tensile strength of about
5.7 ksi.
The below table (Table 1) summarizes the results discussed above
for the Sample A and Sample B SLBC systems as compared to a
conventional bi-layer bond coat:
TABLE-US-00001 TABLE 1 BC BC TBC Dep Rate Ra Microstructure Tensile
Tensile Sample (mils/pass) (.mu.inch) pass/fail (ksi) (ksi) Sample
A 0.68 490 PASS >12 5.1 Sample B 0.53 452 PASS >12 5.7
Bi-layer .6-.65 418 PASS >12 5.7
The samples of FIGS. 8 and 11 were then tested for TBC endurance in
various furnace cycle tests (FCT) by raising the sample temperature
to 1900.degree. F. (first test) and 2000.degree. F. (second test)
in about 10 minutes in a bottom-loading CM furnace, followed by a
hold period of 0.75 and 20 hrs, respectively, and then cooling to
less than 500.degree. F. in about 9 minutes. The cycle is repeated
until more than 20% of the surface area of the ceramic coating
spalls from the underlying surface. The approximate hours to
failure for the Sample A, Sample B, and comparative Bi-layer sample
are provided in the below table (Table 2):
TABLE-US-00002 TABLE 2 FCT Approximate Hours to failure
1900.degree. F. 2000.degree. F. Sample 0.75 hr 20.0 hr 0.75 hr.
20.0 hr Sample A 1800 2750 240 1400 Sample B 2300 5700 400 1350
Bi-layer 800 5700 400 1300
While the present subject matter has been described in detail with
respect to specific exemplary embodiments and methods thereof, it
will be appreciated that those skilled in the art, upon attaining
an understanding of the foregoing may readily produce alterations
to, variations of, and equivalents to such embodiments.
Accordingly, the scope of the present disclosure is by way of
example rather than by way of limitation, and the subject
disclosure does not preclude inclusion of such modifications,
variations and/or additions to the present subject matter as would
be readily apparent to one of ordinary skill in the art.
* * * * *
References